Plants branched or unbranched; branches 0-9(-17). Roots diffuse; upper portion not enlarged. Stems spheric to cylindric, usually (4-)5-16(-30) × (2.3-)3.5-6.8 cm, firm; tubercles 4.8-12(-15) × 3.5-7 mm; axils appearing naked; cortex and pith not mucilaginous; latex absent. Spines (19-)26-33(-38) per areole, glabrous; radial spines 17-35 per areole whitish or pale tan, bristlelike, 6-12 × 0.1-0.15 mm, stiff; central spines (2-)3-4 per areole, 1-3(-4) spines at least hooked (uncommonly 0 hooked), reddish to purplish brown to almost black (rarely golden brown), abaxial 1 porrect, others inconspicuous, appressed against radial spines, hookless porrect spines (3-)9.5-25 × 0.1-0.5 mm; subcentral spines 1-3 per areole, adaxial to central spines, sometimes transitional to central spines, usually straight and barely distinguishable from radial spines. Flowers ca. 2 × 1.8-3.5(-4.5) cm; outermost tepal margins minutely fringed; inner tepals bright rose-pink or rose-purple, 10-16 × 4-8 mm; stigma lobes yellow-green to green, 3-7 mm. Fruits green, turning bright red, scarlet, or carmine (rarely yellowish), elongating until clavate after color change to red is complete, 12-29 × 5-8 mm, juicy only in fruit walls; floral remnant persistent. Seeds black, 0.8-1 × 0.7-0.9 mm, pitted; testa hard, brittle; anticlinal cell walls straight; interstices conspicuously wider than pit diameters; pits bowl-shaped. 2n = 22.

Flowering Apr-Sep; fruiting Sep-Mar. Chihuahuan and Sonoran desert scrub, grasslands, interior chaparral, oak woodlands, alluvial slopes, hills, canyons, silty, sandy, gravelly, or rocky soils of igneous or calcareous origin; 80-1400 m; Ariz., Calif., N.Mex., Tex.; Mexico (Chihuahua, Sonora).

Mammillaria grahamii is geographically variable. Past attempts to distinguish larger or western individuals as M. microcarpa have proven arbitrary.

All spine hooks on a plant may be oriented in the same direction, a trait sometimes mistakenly said to be limited to Mammillaria mainiae. Plants with short, straight central spines (rarely a mixture of both hooked and straight spines on the same stem) occur in some populations; they have been called M. oliviae or M. grahamii var. oliviae. The name Mammillaria microcarpa Engelmann has been widely used but was not validly published until after M. grahamii.

Benson 1969, Benson 1982, FNA 2003

Common Name: Graham's nipple cactus Duration: Perennial Protected Status: Salvage restriced status in Arizona. General: Low globular cactus that is either branched or unbranched plant with 0-9 branches and diffuse roots, the stems are spheric to cylindric and 5-16 cm tall by 3.5-7 cm wide with tubercles that are 5-12 mm by 3.5-7 mm with axils that appear naked. Spines: Spines 26-33 per areole and glabrous, with 17-35 radial spines that are whitish or pale tan and bristlelike, these 6-12 mm long and less that 1 mm wide and stiff, the 3-4 central spines have 1-3 hooked ones which are reddish to purplish brown to black and the others shorter, less colored and straight. Flowers: Roughly 2 cm by 2 cm with the outermost tepal margins minutely fringed with inner tepals that are bright rose pink to rose purple, 10-16 mm long by 4-8 mm wide, with stigma lobes that are yellow green to green and 3-7 mm. Fruits: Fleshy fruits green turning bright red, scarlet to carmine, they are barrel shaped and elongating until the color change is complete, often with the floral remnants. Ecology: Found on silty, sandy, gravelly, or rocky soils, often on slopes in the chaparral and grasslands from 2,000-5,000 ft (610-1524 m); flowers April-September, fruits September-March. Distribution: s CA, AZ, s NM, TX; south to n MEX. Notes: Distinguished by being erect with tubercles; hooked central spines; no nipples visible; the flowers ringed around towards the top but not at the apex of the 4-7 cm wide stem; and the 2.5 cm wide pink tepals which are not or only minutely fringed, and are followed by elongated bright red fruits. There are quite a few varieties, at least historically. The systematics according to FNA seem to make this a geographically variable species, rather than making species distinctions. Benson 1969 listed two varieties. Ethnobotany: The dried fruit was cooked and eaten, as was the raw fruit, it was boiled and placed warm in the ear for earaches, and the raw pulp was eaten, primarily by children as a snack food. Etymology: Mammillaria from the Latin mammilla, a nipple, while grahamii is named for James Duncan Graham (1799-1865), he was the astronomer for the survey of the final boundary between Mexico and the United States in 1851. Synonyms: Mammillaria grahamii var. grahamii, Mammillaria grahamii var. oliviae, Mammillaria microcarpa, Mammillaria microcarpai var. auricarpa, Mammillaria milleri, Mammillaria oliviae Editor: SBuckley 2010, FSCoburn 2015